| Objective:To investigate the treatment effect of Damon system used in non-extraction crowding cases, cephalometric analysis and dental arch width measurements were conducted before and after the treatment, and the changes compared before and after treatment.Material and Methods:Forty-two consecutively treated skeletal class I crowding cases were included. Thirty-six cases crowded in maxillary were divided into different groups according to space discrepancy(SD):group U1(Upper arch SD<5mm,18 cases), group U2(Upper arch SD>5mm,18cases). Thirty-six cases crowded in mandibular were divided into different groups according to space discrepancy(SD):group L1 (lower SD<5mm,18 cases), L2 group(lower SD>5mm,18cases). According to whether the eruption of the second molar or not, the U2 group was further divided into the second molar eruption group(G1 group,9cases)and second molar no-eruption group(G2 group,9cases).Before and after treatment the dental arch width and the cephalometric measurements were collected for each group and the t test was then compared. Correlation Analysis and Regression analysis was done in order to study the related factors of the crowding cases treated with the Damon self-ligating system.Results:U1 group after treatment, the width between the second molar (WU5)increased by 1.18mm, the width between the first molar (WU6) increased by 1.14mm. For the U2 group after treatment, WU4 increased by 2.41mm, WU5 increased by 2.43mm, WU6 increased by 1.72mm. U1-NA increased by 2.08mm. Gl group after treatment, WU4 increased by 2.26mm, WU5 increased by 2.27mm. With the G2 group, after treatment, WU4 increased by 2.57mm, WU5 increased by 2.6mm, WU6 increased by 2.0mm; U1-NA increased by 2.05mm. The L1 group, after treatment, increased canine width (WL3) by 0.76mm, the width between the first premolar (WL4) increased by 3.52mm, the width between the second premolar (WL5) increased by 2.39mm, the width between the first molar (WL6) increased by 1.94mm, and there was a 4.89 degree increase in incisor inclination, as well as an increase in the Ll-NB group by 1.58mm. The results for the L2 group after treatment, was that WL3 increased by 3.61mm, WL4 increased by 2.48mm, and WL6 increased by 1.72mm. There was a 7.83 degree increase in the incisor inclination and L1-NB increased by 2.89mm. The differences were statistically significant between each group both before and after treatment (P<0.05).The correlation analysis indicated the lifting of the upper crowding arch is related significantly with the width changes of the first premolar, the second premolar, the first molar. The incisor inclination (UI-SN) and UI-NA are also the related factors.The correlation analysis indicated the lifting of the mandibular crowding arch is related significantly with the width changes of the canine, the first premolar, the second premolar. The incisor inclination (LI-SN) and LI-NB are also the related factors.Regression analysis showed that selected model variables for maxillary crowding were the first premolar arch width difference(CWU4),and the incisor inclination difference (CUI-SN), both before and after treatment. The total coefficient is 79.92%. Select model variables for mandibular crowding were the canine arch width difference(CWL3),the first premolar arch width difference(CWL4),both before and after treatment, and the CLI-NB difference. The total coefficient is 75.32%.Conclusion:1. Slight crowding of the upper arch is mainly corrected by changes in arch width, and not the anterior lip inclination trend when crowding cases were treated with Damon self-ligating system.2. On the upper arch moderate to severe dental crowding is corrected by changes in anterior lip inclination and arch width when crowding cases were treated with Damon self-ligating system.3. The expansion of mandibular arch crowding is not only reflected in the post-arch, but also embodied in the former arch, and accompanied by the tilting of the anterior lip when crowding cases were treated with Damon self-ligating system.4. If the second molar is not erupted, application of arch expansion techniques should be done as early as possible in order to help reduce the degree of lip tilting by increasing the arch width.
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